3 X 5 Card Optical Focus Technique
At this point in the process, both beam and optical focus should be
excellent, but further refinement is sometimes possible.
Use a plain white 3 x 5 card for finding the exact focal distance of the
projector. Do this by moving the card fore and aft in front of your screen to
see at where a fine focus pattern is best in focus. If it is already exactly at
the screen surface plane throughout the screen, then you are done. If it is more
than 1 cm in front of or behind the screen do the following. And yes, this will
temporarily undo the hard work done getting the phosphor grain sharp.
Display a fine detail focus pattern (internal focus pattern or S&V Home
Theater Tune-Up) and intentionally overfocus the optics (rear lens control) so
that the center is best optically focused about 2 cm short of the screen. This
is in the direction that extends the lens barrel forward. Bringing the focal
plane slightly short of the screen lets you more easily examine the focal
distances throughout the image area. Check the focal distance for each screen
edge by moving the 3 x 5 card back and forth in front of the screen. You'll be
able to see very accurately the distance at which things are best focused on the
3 x 5 card. Check if the distances are uneven between left/right (indicating a
horizontal lens flapping error) and top/bottom (indicating a vertical lens
flapping problem). Fine tune lens flapping to make the focal distances for top =
bottom, and left = right. This is the time to fine tune lens flapping rings. The
3 x 5 card check is so precise that you'll notice the flapping changes caused by
uneven tension on the lens mounting screws.
Next, pay attention to how the left and right edge distances compare to the
distance at screen center. They should be about equal between center and edges.
If not, slightly adjust the inner and outer optical focus to bring both the
edges and center to focus about 2 cm in front of the screen. Notice that I don't
have you check the screen corners. That is because there will almost always be a
difference in the extreme corners and center and using the left/right edges
gives a good compromise which preserves the central focus where the video image
is going to be sharpest portion of a movie frame anyway.
At this point the optics are perfectly balanced in terms of Scheimpflug and
inner vs outer lens focus. The next step is to shift the entire optical plane
back onto the screen surface. Make tiny movements of the rear lens focus knob
while repeatedly checking the optical focus position with the 3 x 5 card. One
thing to consider is that you probably did all this with the projectors lens
hood off and the lenses are a little cooler than normal. As the lenses warm up,
their index of refraction decreases, the lens mounts expand, and the focal plane
can move slightly inward or outward. You should recheck the focal plane when the
optics reach normal operating temperature an hour or so after the lens hood and
hushbox are closed. Also, recheck 24 hours later and compensate for mechanical
settling.
If you use the 3 x 5 card for final optical focus, you'll note that it is so
sensitive an indicator that even the slight shift of the lens while tightening
the focus knobs will be detectable. Focusing a projector is an iterative process
in which improvement in either optical or beam focus enable finer observation
and adjustment of the other focus system. Patient and diligent technique is
needed to achieve maximal sharpness of both optical and beam focus systems.
Expect a great deal of exercise moving between screen and projector using this
technique.
Perfected optical focus means that even better beam focus may be possible.
Revisit beam focus and fine tune as appropriate.
Only AFTER both optical and beam focus are perfected should you do final work
on geometry and convergence can proceed. Otherwise, geometry and convergences
changes wrought by focus adjustments will upset a hard gained geometry and
convergence setup.
Speaking of which, the next steps in projector setup would be:
Rough geometry and convergence
Grayscale calibration (with possible blue gun defocusing)
Final geometry and convergence
(These steps are not discussed in this article).
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